It is well known that it is desirable to provide security for various types of articles. For example, items of value such as clothing or other goods may be stolen from a retail store environment by simply placing them in a shopping trolley and pushing the trolley from the store without payment, a form of crime known as a “push-out” or “push-through”.
The primary form of deterrent in common use in such environments is the Electronic Article Surveillance (EAS) system, for example those manufactured by the Sensormatic Electronics Corporation (for example see U.S. Pat. No. 3,895,368: “Surveillance system and method utilising both electrostatic and electromagnetic fields”). In these systems, some form of passive (unpowered) tag is attached to the item to be guarded and normally removed or deactivated at the checkout till. If the item is not taken to a checkout then the tag is not removed, and its presence is detected by loops at the store exit, causing an alarm. The loops generate an alternating electromagnetic and/or electrostatic field with particular characteristics, the presence of a tag then causing perturbations in this field which are detected by the loop, causing an alarm to sound to warn security staff that a theft is occurring.
Various tag-construction and tag-detection methods are employed by these systems, operating at various frequencies from several 100 Hz to 100 kHz and above. An example is acousto-magnetic detection whereby a magnetic material in the tag is caused to resonate by a short burst of excitation from the loops, and this resonance then induces back a small “ringing” current in the loops which causes the alarm to sound. Alternatively the tag may contain a diode which causes re-radiation of received energy at harmonic frequencies, this again being detected by the loops. Other methods are used also. The precise mechanism of these systems is not relevant to the present invention, the important feature being that all such systems use one or more loops located at the store exits which transmit a detectable AC electromagnetic field with defined characteristics.
It is known from DE 195 29 456 to provide transponders or barcodes on individual supermarket trolleys and to monitor movement of the trolleys about a store by interrogating the trolleys electromagnetically or by way of a barcode reader at predetermined locations. This allows an alarm to be generated if a trolley is detected to have been pushed into a store and then out again without passing through a checkout. This system makes no use of existing EAS infrastructures.
A more sophisticated system is known from the present applicant's co-pending International patent application WO 01/27891. There is disclosed a security system for protecting articles, the system comprising at least one Electronic Article Surveillance (EAS) loop, one Emitting Unit and one Detecting Unit attached to each article to be protected (generally a shopping trolley), the Detecting Unit having a detector to detect electromagnetic fields emitted by the EAS loop and the Emitting Unit and to distinguish between them, and an alarm generator for generating an alarm if a predetermined sequence of fields is detected. The system combines existing EAS loops normally present in a retail store with Emitting Units and Detecting Units so as to monitor movement of articles, especially supermarket trolleys, around the retail store. Each Detecting Unit may provide two-way communication with the Emitting Unit and thereby to provide information regarding a location and/or status of the Detecting Unit. Accordingly, each Detecting Unit may require relatively complex and expensive electronics.
WO 98/46469 discloses a security system for preventing theft of shopping trolleys and the like. One wheel of each trolley is provided with a selectively activatable braking mechanism, the mechanism being activated when the trolley passes over a signal cable configured as a loop that generally surrounds a retail environment (e.g. the grounds of a supermarket including its car parking area). The braking mechanism is activated by simply detecting an electromagnetic field generated by the signal cable. The system, however, does not provide any way of monitoring individual trolleys or their locations. Similar systems are disclosed in U.S. Pat. Nos. 6,037,869, 6,127,927, 6,353,388 and 6,362,728.